This invention relates to thermal inkjet printing and more particularly to the energizing of heater resistors within an inkjet printhead to expel ink.
A thermal inkjet printhead comprises one or more ink-filled channels communicating with an ink supply chamber or cartridge at one end and having an opening at the opposite end, referred to as a nozzle. A heater resistor is located in the channel at a predetermined distance underneath the nozzle. The resistors are individually addressed with a current pulse to momentarily vaporize the ink to form a bubble. The bubble expels an ink droplet towards a recording medium such as paper. By energizing heater resistors in different combinations as the printhead moves across the paper, an inkjet printer prints different characters on the paper.
The heater resistors within the printhead are addressed through flexible conductors that connect the resistors to control circuitry within the thermal inkjet printer. In many prior systems, each resistor is connected directly to a flexible conductor. For a printer with relatively few resistors, this is a simple and efficient scheme. The base of an inkjet cartridge is large enough to accommodate the printhead as well as tab tape that holds conductive leads connecting each resistor to a flexible conductor. However, such printers print relatively slowly because the few resistors provide a narrow printing swath and have relatively poor resolution because the resistors provide few dots per inch (dpi). The number of resistors can be increased to some degree by increasing the number of individual conductive leads that may fit on the area of the cartridge base. But the process for doing so requires precise methods for reducing the width of the leads and their accurate placement on the tab tape, and is thus expensive.
An alternative to direct connection is multiplexing of the flexible conductors to reduce their number. With multiplexing, the output of a number of flexible conductor determines which resistors are to be heated. Referring to FIG. 1, there is shown a multiplexing scheme employed in U.S. Pat. No. 4,887,098. Logic control circuitry 14 in the printhead decodes the output of three flexible conductors for determining which heater resistor is to be energized. The outputs of the control circuitry 14 are connected directly to NMOS transistors that act as a drivers for controlling the current and thus the energy delivered to the heater resistors. Such gate transistors are required because typical logic control circuitry is not designed to source sufficient current for delivering sufficient energy to the heater resistors. The NMOS transistors enable the heater resistors to draw the needed energy from the power supply. With this scheme, up to eight resistors can be controlled through the three flexible conductors, greatly reducing the number of conductive leads required on the cartridge base.
Integrating transistors such as these NMOS gates into a printhead, however, introduces problems not present in the prior printers that employed direct connections. The characteristics of individual transistors may vary due to different mobilities over the process skew, variation in gate length, oxide thickness, etc. In addition, the voltages supplied to the transistor and the ambient temperature around the transistor may vary. These factors combine to cause fluctuations in the transistor output voltage and thus the amount of energy delivered to the heater resistor. The result is inconsistent print quality.